When the list of indexed TERMS is known in advance, what you have to do is to go back to the KEYS, and then format your IndexMatic queries so that they capture these keys and output those terms.

If the final index entries have the form <LastName>, <FirstName> (e.g, Jobs, Steve) it may be assumed as a first approximation that querying <LastName> (i.e, Jobs) is sufficient to identify a match. Of course—as the very example of "Jobs" shows—this strategy may still lead to homonymic ambiguities, so manually refining some queries remains a necessity.

Now, to convert the initial list into a clean query list, the method I suggest is to replace each
    <LastName>, <FirstName>
    <LastName> => <LastName>, <FirstName>

Noting that $0 always represents the match, you can even shorten the query as follows:

<LastName> => $0, <FirstName>

Note: When dealing with proper nouns, do not forget to force case sensitivity in IndexMatic.

But, how do you quickly transform the original list into the desired format? Easy! Execute Find/Change in InDesign and use the following GREP:

Find what:  ([^,]+)(.+)
Change to:  $1 => \$0$2

Then your query list is ready to run in IndexMatic:

How to manage 'LastName, FirstName' queries from InDesign to IndexMatic.

• See also:
IndexMatic Manual (PDF);
IndexMatic | Frequently Asked Questions.